Fossil fuels available on earth are limited. The most convenient form of fossil energy is liquid hydrocarbon. Coal, which is a solid hydrocarbon, must be mined. Furthermore, it is difficult to refine coal to remove sulphur. The result is that the combustion of sulphur-bearing coal produces sulphur oxides which introduce acid rain into the earth's atmosphere.
Power plants designed to utilize nuclear fission will produce energy. However, there is always the risk of a nuclear accident which releases ionizing radiations to the atmosphere. Furthermore, there is the problem of disposing of nuclear wastes. Physicists have long realized that the sun is a nuclear power plant which produces an enormous quantity of radiant energy. Less than a billionth of this energy is intercepted by the earth, and only about 60 percent passes through the atmosphere to warm the earth's surface. If it were possible to utilize 10 percent of the radiant energy from the sun, impacting on a plot of land having an area of approximately 8800 square miles, the energy achieved would equal the electrical power generated by all of the power plants in the United States.
Before the art of solid-state physics had reached its present level, scientists had attempted to generate power by converting sunlight to heat, using the heat to generate steam and, ultimately, employ the steam to drive electrical generators. It is now possible to convert sunlight directly into electricity by solar cells. These devices are advantageous for a number of reasons. First, they are non-polluting; second, they have a potentially long life; and third, they require minimum maintenance and have no moving parts. The physical principles of solar cells are well known. They have been reliably used in the space program; they have been used in radio-relay stations, navigational aids, and intrusion alarms; and they have been used even to power watches. The commercial use of solar cells is greatly limited because of the cost of the cells.
All solar cells are essentially solid-state rectifiers. A semiconductor-wafer may be doped with a small concentration of dopant so that it will conduct electricity. Most semiconductors, such as silicon, are poor conductors. Depending on the nature of the dopant, silicon will conduct negative charges (electrons) or positive charges (holes). When radiant energy from the sun strikes a solar cell made of silicon, the electrons will move towards the n-type silicon and the holes toward the p-type silicon. The separation of the two charges will produce a voltage, roughly, of about 0.5 volt. While the voltage produced is independent of the area, the current produced is proportional to the area. The efficiency of a typical silicon solar cell lies between 10 and 18 percent.
There are many types of solar cells. Semiconductors, which may be doped either p-type or n-type, may be made of a single crystal such as Si or GaAs. Polycrystalline semiconductors may involve solar cells formed of heterojunctions, such as p-CuInSe.sub.2 /n-CdS.
1. Field of the Invention
Our invention relates to solar cells--that is, photovoltaic devices adapted to transform photons directly into electrical potential.
2. Description of the Prior Art
The first solar cell was invented by Chapin, Fuller, and Pearson (U.S. Pat. No. 2,780,765). This disclosed a p-n junction in single-crystal silicon. These devices form the basis of solar cells in commercial use today. The limitation to the growth of use of solar cells is cost. The present cost of a silicon solar cell is approximately ten dollars per peak watt. To manufacture a silicon solar cell requires the reduction of sand (silicon dioxide) into metallurgical silicon. This metallurgical silicon must then be converted into an intermediate compound, such as a silane, which must then be purified. The intermediate compound--as, for example, trichlorosilane--must be converted into polycrystalline silicon, which must be very pure. The purified silicon is then melted at 1410.degree. C. and crystallized by the Czochralski method. This embodies contacting the melt with a small seed of a single crystal, followed by adjustment of the temperature to initiate growth. The growing crystal is then pulled away from the molten silicon as it grows, gradually consuming the silicon. The silicon is continuously replenished in crystal-growing apparatus now in use. The result is a single crystal which is typically 4 inches in diameter and 3 feet long. The individual silicon cells are then prepared by slicing the crystal into wafers 4 inches in diameter and 0.02 inch thick. Approximately one-third to one-half of the expensively acquired silicon is thus lost in costly sawdust. These wafers are then processed to produce a p-n junction and complete the device.
An alternate method of preparation is to pull a flat ribbon of silicon from the melt through a graphite die (U.S. Pat. No. 4,036,666 to Mlavsky). The thin-ribbon method reduces the cost, owing to the saving of time, energy, and material. However, there is some loss in crystal quality, which results in a less efficient cell. Then, too, the cost remains relatively high, since purification and the high-temperature melting steps are still present in the process.
A lower-cost type of solar cell employs thin films, in which the p-region and the n-region are applied to a substrate which furnishes the structural support for the thin film. A comprehensive review of thin-film research is presented in the Proceedings of the 16th I.E.E.E. Photovoltaic Specialists Conference (1982), beginning at page 840 and extending through page 845. In contrast to the bulk of a silicon wafer (0.02 inch thick), thin-film solar cells provide p- and n-regions which are only 0.0005 inch thick or less. This requires that the films be deposited on a substrate, which may be made of metal or metal-coated ceramic. One example of a thin-film solar cell is disclosed in U.S. Pat. No. 3,416,956 to Keramidas et al. Cadmium sulphide (CdS) is deposited on a substrate by vacuum evaporation, and copper sulphide (Cu.sub.2 S) is formed by ion exchange with a cuprous chloride (Cu.sub.2 Cl.sub.2) solution. The resulting CdS/Cu.sub.2 S layers form a heterojunction cell which may have more than a 10-percent photovoltaic efficiency. However, this cell is chemically unstable and will degrade within hours to months, depending on its encapsulation. More suitable thin-film solar cells are n-CdS/p-CdTe. These solar cells, however, suffer from contact degradation of the CdTe layer. Another thin-film solar cell, n-CdS/p-CuInSe.sub.2, has been made to produce a 10-percent solar efficiency and demonstrate stability. However, this cell suffers the disadvantage of being very expensive to prepare, since a three-source vacuum evaporation technique is required. A very different thin-film cell is fabricated from amorphous silicon. This, however, requires the same expensive silicon purification process. There is a saving in cost which ensues because of the thin film used--namely, a layer of thickness of 2.4.times.10.sup.-5 inches. The advantages, however, are offset because amorphous silicon cells suffer a 10-percent degradation in the very first month of their use.
Cadmium sulphide, as an n-layer of thin-film cells, has been studied in depth, since CdS is a very well-understood semiconductor which has excellent electrical properties in the thin-film form. However, it must be combined with a second layer that is p-type and is an efficient absorber of sunlight. This is owing to the fact that CdS has a wide band gap and absorbs little of the solar spectrum. Crystalline silicon also has a relatively low absorption coefficient and hence requires a layer which is at least 0.02 inch thick so that sufficient sunlight will be absorbed. CuInSe.sub.2, CdTe, and Cu.sub.2 S are very strong absorbers across the entire solar spectrum. The advantage of this is that the photons of sunlight are all absorbed in a very thin layer of less than 4.times.10.sup.-5 inches. Consequently, the resultant electrons and holes do not have a long distance to travel to reach the electrode. Since thin films are of poor crystalline quality relative to single-crystal silicon, it is important to have the shortest possible path for the electrons or holes. Otherwise, many of these carriers are lost at the crystalline imperfections that are present in thin films that are comprised of very small crystallites. The boundary between grains is a very significant imperfection in this sense, and the thinner the layer, the fewer grain boundaries that must be traversed.
All of the thin-film solar cells discussed above have been heterojunctions which are formed between two semiconductor regions that are chemically different. There are two major advantages of a heterojunction. If one region has a greater band gap, as CdS does in a CuInSe.sub.2 /CdS cell, it absorbs little of the sunlight. Thus, light incident on the CdS side efficiently reaches the interface where, due to the small absorption depth of the p-region CuInSe.sub.2, it is likely that absorption will occur, and the resulting electron-hole pairs are separated by the field that is created by the junction. In some semiconductors, such as CdS, a p-n junction can only be formed by a heterojunction because it can only be doped n-type. The charge-transport properties of CdS are very good in thin films, and it has been used in combination with a p-type semiconductor of the desired 1.0-1.6 e.v. band gap. A solar cell employing a heterojunction suffers from the discontinuity between the two layers. There is interference with charge transport, owing to the presence of traps or recombination sites for carriers. Another problem is a conduction-band discontinuity or spike, or both, which results from the differences in the electron affinities of the two semiconductors. The electron affinity is measured from the bottom of the conduction band to the vacuum level, which is the energy level above which an electron can escape from the solid. If the junction is improperly designed and the electron affinities of the two semiconductors are not closely matched, then barriers or spikes may exist in either the valence band or the conduction band, or both. It is believed that these features will decrease the efficiency of a heterojunction solar cell.